Coaxial stub tuner



Jan. 17, 1961 l ET G. BooK COAXIAL STUB TUNER Filed Aug. 1o, 195e 2sheets-sheet 1 Jan. 17, 1961 E. G. BooK 2,968,776

COAXIAL. sTUB TUNER Y IN1/EN TOR.

United States Patent O COAXIAL STUB TUNER Eric G. Book, Mokena, Ill.,assignor to Andrew Corporation, a corporation of Illinois Filed Aug. 10,1956, Ser. N0. 603,437

4 Claims. (Cl. 333-82) rllhe present invention relates to a fine tuningmethod particularly applicable to coaxial stub filters.

Heretofore, coaxial stub filters and various arrangements thereof,referred to as diplexers, duplexers, and the like,'h'ave been diicult totune. It has been common to employ annular shorting plates provided withspring contact fingers for retaining them in position, or they have beensoft soldered into position. Often times in attempting to fix suchshorting plates into position there has been some change so that theoptimum tuning has not been accomplished.

It, therefore, would be desirable to provide some means to facilitatetuning coaxial stub filters or similar structures. In accordance withthe present invention it is proposed to provide coarse or fine orvernier tuning in an improved assembly.

I It is, therefore, an object of the present invention to provide aconvenient means to facilitate tuning coaxial stub filters.

Another object of the invention is to provide a Vernier tuning means forcoaxial devices to vary the characteristie impedance thereof.

Still another object of the invention is to provide a tuning means forcoaxial devices having coarse and fine tuning means.

Other and further objects of the invention subsequently will becomeapparent by reference to the following description taken in conjunctionwith the accompanying drawings, wherein:

Figure 1 shows the invention applied to a typical duplexer or diplexeras seen in longitudinal cross-section;

Figure 2 is a view as seen in the direction of the arrows along the line2 2 of Figure 1;

Figure 3 is a view seen in the direction of the arrows along the line 33 of Figure 2;

Figure 4 illustrates another embodiment of the invention as seen inlongitudinal cross-section;

Figure 5 is a diagrammatic representation of the tuning devices shown inFigures 1, 3 and 4; and

Figure 6 is Va curve illustrating the characteristics of the inventionrelative to the diagrammatic representation shown in Figure 5.

The arrangement shown in Figure l represents a conventional duplexer ordiplexer having a plurality of coaxial line sections, some of which areprovided with coupling flanges for connection to other coaxial lines.For example, the arrangement shown in Figure 1 may have a couplingflange 11 arranged to couple the device to another coaxial line whichmight be connected to a transmitter. The intermediate coupling flange 12connects the device to a coaxial line leading to the antenna element perse, while a third coupling ange 13 connects the device to anothercoaxial line which is either connected to a receiver or to a secondtransmitter. These coaxial devices comprise an outer conductor` 14connected to the flanges 11, 12 and 13, and have inner coaxialconductors 15. Located at right angles to the interconnecting innerconductor 16 are two similarly arranged tuning stubs or devices, onlyone of which needs to be described. A transversely arranged innercoaxial member 17 is connected to the transverse conductor 16. Adjacentthe end of the conductor 17 and its outer conductor 14 is an annularring 18 which is held in adjustment by a clamp ring 19 surrounding theoutside of the outer conductor 14. The annular ring 18 is provided withVernier tuning devices which are best seen by referencel to Figures 2and 3. The annular shorting member 18 is provided with a slit 21 whichextends past the center from one annular edge. The shorting member 18 isoriginally adjusted to `approximately the proper tuning position.Thereupon the clamp ring 19 is tightened. The annular shorting member 18along one diameter thereof carries two threaded rods 22 and 23 whichengage threaded openings in the annular member 18. Suitable lock washersand lock nuts 24 and 25 are provided for each threaded rod 23. Theeffect of the threaded rods is to alter the characteristic impedance ofthe portion of the line in which they are located, thereby providing avariable length of coaxial line that has a different impedance. As

the rods are extended into the space between the inner and outerconductors 14 and 17 comprising the cavity to be tuned, the electricallength thereof will be shortened. The rods are of a diameter at leastequal to 1/4 the space between the inner and outer conductors.

In Figure 4 there is shown a coaxial transmission line having an innerconductor 31 and an outer conductor 32. A suitable tuning device ismounted at one end of these coaxial conductors, which includes anannular shorting member 33 having a central opening 34 for engagementwith the central conductor 31. The annular ring 33 is secured to acylindrical cap 35, which is adjusted to approximately resonant positionand soldered into position at 36. Two longitudinally movable tuning rods37 and 38 are connected together by a transverse member 39 so that theserods can be moved simultaneously. The position of the rods 37 and 38upon proper adjustment may be fixed by suitable set screws 40 and 41.Where the rods of this kind extend an appreciable distance into acoaxial line, such as the conductors 31 and 32, it is sometimesdesirable to insure the positioning of the free or inner ends of therods 37 and 38 so that they at -all times will be properly spacedsubstantially midway between the inner and outer conductors 31 and 32.Thus, in the arrangement shown in Figure 4 the inner ends of the rods 37and 38 are connected to an annular ring 42 of insulating materialsecured in position to engage the inner and outer conductors 31 and 32.The ends of the rods 37 and 38 are provided with threaded stud members43 which are engaged by two lock nuts 44. Thus, Figure 4 illustratesanother embodiment of a tuning device having the same function as thetuning device shown in Figures l, 2 and 3.

In Figure 5 is diagrammatically shown a tuning arrangement for coaxiallines having an inner conductor A and an outer conductor B and anannular shorting member C provided with two adjustable rods D. Toillustrate the characteristic operation of the invention, the curve inFigure 6 shows a range covered when a tuning device such as thatdiagrammatically shown in Figure 5 was used to tune a second harmonicfilter for an FM stage. Thus, the curve 6 shows the relation betweenfrequency in megacycles and the distance of insertion of a pair of rodsD. It is believed that no further explanation of the operation isrequired in view of this graphical representation in Figures 5 and 6.

Those skilled in the art will appreciate that in some instances a singleadjustable rod may suice, and that in other cases more than two rods maybe used at different radii. While the rods have been shown midwaybetween the inner land outer conductors, some latitude is possible inpositioning the rods closer to one of the coaxialcon ductors than to theother conductor.

While Yfor the purpose of .illustrating land describing the presentinvention certain particular embodiments 'have been shown in thedrawings, it is to be understood that the invention is not to be limitedthereby since such modications or variations are contemplated as may becommensurate with the spirit and scope of the invention ys'et forth inthe accompanying claims.

I claim as my invention:

1. A tunable coaxial stub filter comprising a coaxial -line havingspaced inner and outer coaxial conductors and a coaxial half-wavelengthstub having inner and outer conductors forming a longitudinally uniformannulus yand terminating at one end on the inner and outer conductors ofthe line, respectively, the other end having an annular conducting plateextending between the inner and outer conductors, and longitudinallyslideable in the coaxial cavity thus formed, a plurality of conductingrods of transverse dimension equal to `at least 1A the thickness of theannulus carried by said plate and extending perpendicularly through saidplate approximately midway between the conductors and parallel therewithand adjustable in longitudinal position in the plate, the annulus beingotherwise free of conducting elements, means for locking thelongitudinal position of the plate for coarse tuning of the stub, andmeans for locking the longitudinal position of the rod in the `plate forfme tuning of the stub, sothat the half-wave resonant frequency of thestub is increased in the rst portion of the insertion of the rods.

2. A tunable coaxial stub filter compirsiug a coaxial line having spacedinnerv and outer coaxial conductors and "a coaxial half-wavelength stubhaving inner and outer conductors forming va longitudinally uniformannulus and terminating at one end on the inner and outer conductors ofthe line respectively, the other end having a shorting member connectingthe inner and outer conductors, a plurality of conducting rods oftransverse dimension equal to at least 1A; the thickness of the annulusextending through the shorting member into the annulus approximatelymidway between the inner and outer conductors, the annulus beingotherwise free of conducting elements, and means to adjust thelongitudinal position of the rods to tune the stub so that the half-waveresonant frequency of the stub is increased in the tirst portion of theinsertion of the rods.

3. A tunable coaxialstublter as set forth in claim 2 wherein theadjusting meanscomprises a connecting member aiiixed to the outwardlyextending ends of the rods for simultaneous adjustment of the rods.

4. A tunable coaxial stub lter as set forth in claim 2 having an annularinsulating ring within the stub, the inner ends of the rods being seatedin the insulating ring to maintain their relative positions.

References Cited in the le of this patent UNITED vSTAT ES PATENTS2,203,806 Wolf lune l1, 1940 2,412,161 Patterson Dec. `3, 1946 2,454,062Holman et al Nov. 16, 1948 2,500,430 Pierce Mar. 14, 1950 2,523,725Schmidt Sept. 26, 1950 2,558,463 Reed June 26, 1951 2,593,183 Rado Apr.15, 1952 2,725,537 Barrow Nov. 29, 1955 `2,743,422 Muchmore Apr. `24,1.956 2,757,314 Sheppard July 31, 1956 2,773,215 Miller Dec. 4, 19562,806,138 Hopper Sept. l0, 17957

